Joe Taylor wrote:
>
> Dave / W6TE wrote:
>
> > It's my understanding that PC clock stability drifts too much to be useful
> > for some programs that use synchronization between stations (JT44). So
> > Dimention4, I believe, is not useful for these applications. I found new
> > Motorola GPS boards on EBAY for $20.00 they interface with the RS-232 port
> > using available freeware. I have purchased two of the boards but have not
> > finished the interfacing as of yet. Another source of precise clocking is
> > the recent availability of the HP Z3801 disciplined GPS clocks. They are
> > available for around $250 and are complete with internal double ovened
> > oscillator. It needs an antenna and a 50 VDC power supply to run.
> >
> > Dave / W6TE
PC clock stability is inherently poor - a lot of it due to the fact there are
large temperature variations over time. I don't know what JT44 is, and hence
what timing accuracy is required to use it. However, its easy to achieve 100 ms
accuracy using Network Time Protocol (NTP) on a computer as a software solution.
NTP makes uses of accurate clocks (some of which are caesium based) via the
internet. The round-trip delay in getting the information via the internet is
estimated fairly accurately. Several NTP servers are then used to give improved
accuracy, with NTP taking into accout the jitter on the data from different
clocks and attaching more weight to those that have the lowest jitter. I've only
experience of using NTP on Unix systems, but I know PC solutions exist.
This may not be sufficiently accurate, but its free and easy to use.
Incidentally, recently I had a need to look at the data sheet on a clock chip
used in my computer (Sun Ultra 60) and found that if you know the average drift
per month, you can actually re-program the clock chip to run slower or faster.
Personally I could not be bothered to do this, since NTP does the adjustments
continuously and I suspect there is nothing you could do about it if a PC is
switched on/off during the week, as the temperature would change too much.
--
Dr. David Kirkby,
Senior Research Fellow,
Department of Medical Physics,
University College London,
11-20 Capper St, London, WC1E 6JA.
Tel: 020 7679 6408 Fax: 020 7679 6269
Internal telephone: ext 46408
e-mail davek@medphys.ucl.ac.uk
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